Electrolytic process for producing iron products



Jan. 23, 1951 H. v. TRAsK 2,538,990

ELEOTROLYTIO PROCESS FOR PROOUOING IRON PRODUCTS Filed Aug. 22, 1945 2sheets-sheet 1 ELEOTROLYTIO PROCESS FOR PROOUOING IRON PRODUCTS FiledAug. 22, 1945 v H.Y v. TRAsK 2 Sheets-Sheet 2 Jan. 23, 1951 PatentedJan. 23, 1951 ELECTROLYTIC PROCESS FOR PRODUCING IRON PRODUGTS Harold V.Trask, Cooley, Minnsassignor to Bliel Metals Company, St. Paul, Minn.. acorporation of Ohio Application August 22, 1945, Serial No. 611,947

3 Claims.

This invention relates to the production by electrolysis of readilygrindable iron of such composition that it is particularly adapted foruse, after crushingr and annealing, in cold die pressing operationswhere pure iron in the form of particles havingv equiaxial structure isrequired.

It is an object of myinvention to provide an iron powder which issuperior to known products of this kind in structure and composition foruse in iron powder metallurgy.

Another object is to produc-e a low cost iron deposit which ischaracteried by the presence of a sufficient quantity of the oxide andhydroxide of iron to render the product readily grindable to particlesof sizes suitable for use in iron powder metallurgy and from which thehardening constituents may be readily removed by simple annealingtreatment.

A further and particular object is to provide a novel and economicalprocess for producing brittle iron plate of the character described byelectrolysis from relativelyimpure iron and whereby economies areeffected by reason of the low capital investment required in plant andequipment, ease of control in the commercial production offa uniformproduct andv low power consumption per pound of iron produced.

A still further object is to provide an electrolytically deposited,porous, brittle, dull iron plate which adheres well to the cathodesduring fully and from about 400 to 52,0, and its porosity which gives ita specic gravity ranging from approximately 6.3 to 7.25. By crushing orgrinding it may be reduced to particles of the desired size (usuallyminus 100 mesh) and'of equiaxial structure well adapted for use in ironpowder metallurgy. The dull iron plate may be ground in a ball mill withair separation, or in any other (o1. aofi-1o) suitable grinderorpulverizer at low cost. It may be pulverized. to minus l mesh sizes ina ball mill at the rate of 20 to 25 pounds per 1G() pounds of balls perhour, whereas, ordinary electro-deposited iron pulverizes at a rate offrom 0.25 to 1.0 pounds per 100 pounds of balls per hour. The hardeningimpurities, oxides and hydroxides may be reduced and the resultinggaseous elements together with any chlorine carried over from theelectrolyte may be driven 01T by simple annealing treatment leaving aprod.- uct which is more than 99.5% pure iron. The annealing treatmentpreferably comprises heating the ground product in a hydrogen atmosphereat approximately 800 degrees C. for from one to three hours, dependingon the neness of the product.

According to. my process for making dull iron plate having thecharacteristics hereinbefore described, it is essential that theconditions present in the deposition cells with respect to (l) solutioncomposition, (2) current density of the deposition and (3) temperatureof deposition, shall be controlled and maintained within the limitspresently to be described. The range of permissible values from atechnical standpoint must be further limited to minimize the costofproduction and facilitate control in commercial operations,

Solution composition electrolyte comprising a ferrous chloride solutionhas been found best suited for my purposes. An aqueous solutioncontaining from 2O grams to grains of iron per liter may be used; Thelimits of the solution concentration are interrelated with those of thetemperature and current density of' deposition. For example, the upperlimit of iron concentration, as ferrous chloride, in the solution issomewhat dependent on the lowest temperature which can be economicallymaintained in the cell. If, as in most in- Stallatona it is. mit.economical to keep the deposition` temperature below 15 degrees C. themaximum concentration of iron is approximately .1,25 grams per liter fordull iron plate deposition. It is, however, much more economical to keepthe iron concentration below this ligure and I have .found for mosteconomical operation that the iron in solution should be maintained atapproximately l5 grams per liter. With more dilute solutions it isnecessary to increase the voltage 159 impress a given current density ofdeposition and. this progressively increases the. power 00nsumed perpound of iron deposited. It is feasible,

3 however, to obtain dull iron plate with maximum solution concentrationranging from about 67 to 87 grams of iron per liter where the currentdensities range from about 10 to 40 amperes per square foot andtemperatures at or below 25 degrees C. are maintained in the cells.

A further necessary control involves hydrogen ion concentration of thesolution. Its pH should be maintained between 3.0 and 5.5. A pH lowerthan 3 indicates the presence of excessive acid or ferric chloride andresults in a bright iron cathode deposit which is unsuited for mypurposes and is otherwise not satisfactory because of its poor adherenceto the cathode plates. In practice the pH of my solution naturallyadjusts itself between 5 and 5.5. With pl-I's above 5.5 the solutiontends to hydrolize and a deficiency of iron in solution develops underconditions indicated by substantially higher pH values. The presence offerric chloride in the solution is neither desirable nor necessary forthe functioning of my process. Other additions to the electrolyte, suchas ammonium chloride which has been used heretofore, are alsodetrimental.

Current density In order to produce my dull iron plate economically, thecurrent density between electrodes of the deposition cells must bemaintained between certain values which are interdependent upon theconcentration of iron in solution and` temperature of deposition. Ingeneral, the higher the solution concentration the greater must be thecurrent density at any given temperature within the feasible range. Ashereinbefore indicated, practical limits of the current density are fromabout to 40 amperes per square foot where the solution concentrationranges from a maximum of about 67 to 87 grams of iron per liter ofsolution and where a temperature at or below 25 degrees C. is maintainedin the cell. With lower solution concentrations current at a density aslow as 5 amperes per square foot may be caused to pass between the'electrodes in the cells.

The power consumed per pound of iron deposited increases in directproportion to the current density and in inverse proportion to thetemperature of deposition.

Temperature of deposition In order to produce dull iron plate mosteconomically the temperature of deposition should be maintained betweendegrees and 35 degrees C. and preferably at approximately 25 degrees C.where the economical ranges of current densi- 'ties and solutionconcentrations hereinbefore described are maintained. An unsatisfactory,bright, malleable deposit results when a temperature substantially above40 degrees C. is reached in a cell of the character described.

Cells and electrodes 4 and with ease, l employ flexible cathode startingsheets comprising stainless steel such as that containing approximately1.8% chromium and 8% nickel. Sheets of 1% to 1/8 inch thickness havebeen found to be adequately stiff to remain straight in the cells whileaffording the flexbrittle iron deposit by flexing.

The electrolyte for use in my process may be obtained by dissolvingscrap iron, preferably of low carbon content, in hydrochloric acid. Thisi concentrated solution of ferrous chloride is diluted so that itcontains iron Within the limits hereinbefore described and preferablyabout 75 grams of iron, as ferrous chloride, per liter of solution. ThepI-I of'this solution is maintained between 3 to 5.5 as indicated.

To control the temperature of deposition, inexpensive heat exchangersmay be placed in the cells or built into the walls of the same, or theelectrolyte may be circulated through a heat exchanger locatedexteriorlyof the cells.

To illustrate my invention and not by way 0f limitation, reference ishad to the accompanying drawings in which:

Figure l is a diagrammatic plan view of a suitable deposition cell;

Fig. Z is a diagrammatic vertical section through the same;

Fig. 3 illustrates diagrammatically another arrangement of electrodes ina cell, together with means for filtering and cooling the electrolyte ina circuit exterior of the cell;

Fig. 4 is a graph showing the maximum solution concentrations whichlproduce dull iron plate with various current densities and with a celltemperature of 25 degrees C;

Fig. 5 is a graph showing the maximum solution concentrations for dulliron plate with various cell temperatures and a current density of 27amperes per square foot;

Fig. 6 is a graph showing the minimum current densities which producedull iron plate at various temperatures where a solution concentrationis maintained at approximately 75 grams of iron per liter of solution;

Fig. 7 shows graphs indicating the relation of power consumption tocurrent density with certain solution concentrations and where thetemperature of deposition is maintained at 25 degrees C. in producing mydull iron plate;

Fig, 8 shows a graph illustrating the effect of changes in solutionconcentration on power consumption in cells where the current density ismaintained at 3G amperes per square foot'and the temperature ofdeposition at 25 degrees C. in depositing dull iron plate,`and

Fig. 9 shows graphs illustrating the relation between temperature ofdeposition and power consumption with certain solution concentrationsand current density maintained at 30 a'mperes per square foot in dulliron plate deposition.

Referring to Figs. l and 2, the numeral 9 indicates a simple depositioncell containing iron anode plates I5, stainless steel cathode startingsheets II and heat exchangers I2, I3 and I4. The electric circuitincludes a bus bar I5 having connectors I6 severally extending to theanodes I@ of a group, the cathode sheets I I of Which have conectors ilextending to a bus bar I8. A second group anode plates I!) haveconnectors I9 extending to the bar I8 and the cathode starting plates lI of this group have connectors 20 extending to a bus bar 2 I. A sourceof direct current attacco 22.: has conductors; 23: and; 24: connectedAto the bus bars 25|.l` and4 I5. respectively.. Anode plates ofapproximately onel inch thickness arev suitable for my purpose andthesef are preferably spaced two inches, center to center, relativetoithe adjacent stainl'ess steel cathode sheets; Withv my preferredelectrolyte solution in the cell. 9 and by causing current to flowbetween electrodes at the density hereinbefore described, I obtain-dulliron plate on the cathode sheets II- with a voltage drop betweenadjoining electrodes I and I-l withiny the range 1'.5 to 2- volts. Theheat exchangers I2, |3- andy I4 are supplied with brine or other coolingmedium so that the temperature of the electrolyte is maintained withinthe limits described.

In the alternate arrangement shown in Fig. 3, the iron anodeplates In'and stainless steel cathode sheets Il are arranged`- in groups asindicatedv in an open cell 25l andthe electrolyte'is fed into one end ofthe cell through a pipe 26 and continuouslywithdrawn from the other endthrough a pipe 251 by means of a pump 28. From the outlet of this pumpthe solution is fed through a pipe 29 supplying aiilt'er 30 whichremoves solid particles of impurities and discharges and clear solutionthrough a pipe 3l extending to a heat exchanger 32. From the latter thecooled electrolyte passes through the pipe 25 back into the cell 25. Acooling medium is` supplied to the heat exchanger 32.

Direct current is supplied from a source 33 havingconductors 34 and 35extending to bus bars 36 and 37 respectively. Four groups of electrodesare shown in series between the bus bars and 31. `The electric circuitincludes additional bus bars 38, 39 and 40 and individual connectors 4Iextending therefrom to the several individual electrodes. Spacing of theelectrodes is as hereinbefore described' with reference to Figs. 1 and2. Other arrangements of the electrodes in the circuit will be obviousto those skilled in the art and it will be evident that in commercialoperations a largenumber of cells are connected either in parallel. orinl series in the circuit and that any suitable source of direct currentmay be provided. For example, the source may comprise a motor. generatorset or where al these sheets are removed from the cell and eXedtodislodge the brittle dull iron deposit after which they may be replacedin the cell to receive a further deposit of iron. Separation of the ironplate from the cathode sheets is facilitated if the coating is allowedto accumulate to a thickness of from 1/8 to 1A inch. The flexing of theiron coated sheets maybe performed manually by bending the sheets overa` roller or bar, or otherwise in a machine designed for the purpose.After repeated use it is sometimes necessary to clean the cathode sheetsbefore returning them to the tank and this may be accomplished bydipping them in dilute hydrochloric acid for a period of from' 1 to 5minutes. Cathode sheets of the character described are so durable thatthey maybe used according to my invention almost indenitely. lTheranodeplates are merely replaced by new ones periodically as they` aredissolved in the electrolyte. Other details oi the operation arewell-known in this art and require no'further explanation.

Figs. 4 to 9 inclusive. showgraphically thefresults-l of a large numberof tests which. I havel conducted to determine the conditionsnecessarytoproduce dull iron plate at low cost. All tests were made with theelectrodes spaced a distance equal to approximately two inches center tocenter in the cells. In Figs. 4 and 5 the curves l2 and 43 represent thehighest solution concentrations which produce dull iron plate under theconditions indicated. The point illv on the curve 42', for example,represents the maximum solution` concentration, 87 grams of iron perliter, from which dull iron plate may be deposited with a currentdensity of 40 amperes per square foot and temperature of- 25 degrees C.I have determined that conditions rep-resented by points below curve 42will produce dull iron plate but that those representedl by points abovethe curve will not at the temperature indicated, namely, 25 degrees C.My preferred solution concentration ('75 grams of iron per liter ofsolution) and current density (30 amperes per square foot)is-represented by a point X below the curve 42. As further indicated inFig. 5, solution concentrations represented by points below the curve43v produce dull iron plate with temperatures below 40 degrees C. andwith current density at 27 amperes per square foot. With thisvcurrentdensity and a temperature of deposition above 40 degrees C. dull ironplate cannot be obtained. Nor can such plate be obtanegl when thesol'u'- tion concentration exceeds about 9U grams of iron per liter andthe deposition temperature is above 20 degrees C.

Graph 45 (Fig. 6) shows the relation between the minimum currentdensities which produce dull iron and the required temperatures wherethe electrolyte contains '75 grains of iron per liter of solution. Thiscurve indicates the minimum current densities required from a technicalstandpoint and, as hereinbefore indicated, the upper limits areestablished by economic considerations.

Points on graphs 45 and 4i', shown in Fig, 7, were obtained bycomputations from power con'- sumption readings in kilowatt hours perpound of iron deposited based on a number of tests using differentcurrent densities and with the' iron concentrations respectively equalto 55 grams per liter for graph 46, and of 78 grams per liter for graph41 and lwith the temperature of deposition at 25 degrees C. The severaltests on which these graphs were based resulted in dull iron deposits.As shown by these graphs the power consumed per pound of product isdirectly proportional to the current density of deposition.

Graph 48 (Fig. 8) `was obtained from tests producing dull iron plate byplotting the power consumption per pound of iron deposit againstsolution concentration in grams of iron per liter of solution. In thesetests constant current density at 30 amperes per square foot anddeposition temperature at 25 degrees C. were maintained. It will beevident that progressively more power is consumed as the solutionconcentration is decreased below 87 grams of iron per liter and that asthe solution concentration is reduced below about 50 grams of iron perliter the rate of power consumption increases rapidly. Under my optimumconditions and with a solution concentration of about '75 gramsof ironper liter, approximately .'75 kilowatt hour is required per pound ofdull iron deposited. With a solution concentra-- tion equal to 25 gramsof iron per liter of solutionV and other conditions unchanged.approximately 1.25 kilowatt hours is required per pound of dull ironplate.

Graphs 49 and 50 (Fig. Q were also obtained from a number of dull ironplate deposition tests and by plotting the power consumption in kilowatthours per pound against various temperatures of deposition with constantcurrent density at 30 amperes per square foot. Solution concentrationsequal to 55 grams of iron per liter were used for graph 49 and of 78grams of iron per liter for graph 50. Upon comparing these graphs itwill be evident that more power is required to obtain a dull irondeposit with the lower solution concentrations. Moreover, the powerconsumed is inversely proportional to the temperature of deposition.

By maintaining the preferred conditions hereinbefore described in thecells, I obtain a porous, brittle, coherent, dull iron deposit withcurrent efficiencies above 100%. The reasons for this amazingly highefficiency are not entirely clear but it is thought that it is at leastin part due to the porosity and coherent nature of my cathode deposit.The outer layer of iron as it is formed may act as an intermediateelectrode between the anode plate and cathode sheet and electrolysis maycause decomposition of water in the pores between this outer layer andthe cathode sheet whereby hydrogen and oxygen are liberated in thesepores. Since the outer layer is iron in a very pure form it may combinewith the liberated gases to form iron oxide and hydroxide under theconditions existing in the cell. Accordingly, current eciencies above100% may indicate that the iron is first deposited and later partiallyaltered to the oxide and hydroxide state without affecting theV carryingpower of the current passing between the anode and cathode. Furthersubstantiation of this theory may be found in the fact that analyses ofthe dull iron plate show that iron constitutes only from 95% to 97.5% ofthe product and that the hardening contaminants are oxide and hydroxidecompounds which can be easily removed by annnealing in a hydrogenatmosphere.

Dull iron plate when deposited under the conditions hereinbeforedescribed adheres to the stainless steel starting sheets well and goodcohesion is obtained as the plate increases in thickness. sheets whenthe latter are flexed as described. These characteristics of the platecontribute in an important degree to economical production.

Under conditions which cause the iron to be pre-- cipitated in the lformof a non-adherent powder at the cathodes it becomes contaminated by thesolid impurities which settle to the bottom of the cell from the anodeplates and it is consequently difficult to remove from the cell anddicult to separate from the impurities. Such contaminated powder is alsosubject to excessive oxidation in the cleaning process.

The use of flexible, stainless steel cathodes containing substantialamounts of chromium and nickel has unique advantages in connection withmy process wherein such cathodes exhibit a critical degree of adherencewith the dull iron deposit which results in even surface accumulation onthe cathodes and subsequent efficiency and economy in the removal of thedeposit by simple flexing. It has been found that where it is attemptedto operate with a pH of solution below 3, the deposit does not adhere tothe stainless steel cathode sheets and causes short circuits due to thepeeling off of the deposit. Moreover, where It also separates readilyfrom the cathode temperatures of deposition above the critical valuesherein described are maintained in a cell containing an electrolyte oflow iron concentration, viz., less than 125 grams of iron per liter ofsolution, a bright iron accumulates on the stainless steel cathode whichis bonded so securely thereto that it cannot be removed economically asby merely llexing and/or scraping treatment. It will thus be evidentthat peculiar and unexpected functional advantages are derived from theuse of flexible, stainless steel cathode sheets according to the presentinvention.

My coherent dull iron plate may be ground at low cost to sizes suitablefor iron powder metallurgy and when nely divided has excellent structurefor this purpose in that the individual particles have substantiallyequiaxial structure rather than a flat structure which results fromattempts to grindbright iron deposits. ther hereinbefore pointed out,the ground product may be puried, without destroying its advantageousequiaxial structure, by simple annealing treatment in a hydrogenatmosphere. Analyses of a number of specimens of my dull iron plate showthe following ranges of composition in percentages by weight:

Percent Total iron as stripped -9'7.5 Chlorine as stripped .3-.6 Weightloss after heating in nitrogen at 950 degrees C. for one hour LGO-1.55Weight loss after heating in hydrogen at 950 degrees C. for one hour2.003.55 Total iron after reduction 99.5-99.9

After the crushing and annealing treatment measured quantities of thepowder may be placed in dies and then pressed to the die shape withoutthe application of heat. Pressures of the order of magnitude of 30 to 60tons per square inch are used. The resulting self-sustaining bodies arethen removed from the dies and subjected to a sintering temperature tounite the component iron particles. The bodies so formed from my producthave great strength, homogeneous structure and other controlledproperties. Test bars formed in this manner from my dull iron plateunder 30 tons per square inch pressure and sintered for 11/2 hours at850 degrees C. have had tensile strength ranging from 23 thousand to 26thousand pounds per square inch and show elongation under test equal tofrom 7 to 10 per cent in one inch. It will be evident that the pure ironpowder may be mixed with other substances to modify the properties ofthe end product.

The present application is in part a continuation of my application,which has been abandoned, Serial No. 560,783, filed October 28, 1944,for patent for Dendritic Iron Product and Process for Making the Same.

Having described my invention, what I claim as new and desire to protectby Letters Patent is:

l. The process for making brittle, porous iron plate byelectrodeposition which comprises, maintaining in a cell a solutioncomposed substantially entirely of ferrous chloride and water at alconcentration within the range 20 to l125 grams of iron per liter andat a pH from 3 to 5.5, subjecting said solution to electrolysis betweena soluble iron anode and a cathode at a current density between 5 and 40amperes per square foot and at a temperature within the range 15 to 40C., the values within 'said ranges being selected to produce a brittle,dull gray deposit having specinc gravity from 6.3 to 7.25.

As fur- 2. The process for making brittle, porous iron plate byelectrodeposition which comprises, maintaining in a cell a solutioncomposed substantially entirely of ferrous chloride and water at aconcentration within the range 67 to v87 grams of iron per liter and ata pH from 3 to 5.5, subjecting said solution to electrolysis between asoluble iron anode and a cathode at a current density between 10 and, 40amperes per square foot and at a temperature within the range 15 to 35C., the values within said ranges being selected toproduce a brittle,dull gray deposit having specific gravity from 6.3 to 7.25.

3. The process for making an iron powder which comprises, maintaining ina cell a solution composed substantially entirely of ferrous chlorideand water at a concentration within the range 20 to 125 grams of ironper liter and at a from 3 to 5.5, subjecting said solution toelectrolysis between a soluble iron anode and a cathode at a currentdensity between 10 and 40 amperes per square foot and at a temperaturewithin the range 15 to 40 C., the values Within said ranges beingselected to produce a brittle, dull gray plate having specific gravityfrom 6.3 to 7.25, pulverizing said plate to particles of sizes andshapes suitable for compaction in forming dies and annealing theresulting powder in a reducing atmosphere and at a low frittingtemperature to produce .a powder containing less than .5% of impurities.

HAROLD V. TRASK.

assae REFERENCES CITED The following references are of record in thefile ofrthis patent:

UNITED STATES PATENTS Number Name Date 1,769,605 Pike' July 1, 19301,912,430 Cain June 6, 1933 2,099,873 Sternfels Nov. 23, 1937 2,157,699Hardy May 9, 1939 2,223,928 Whiteld et al Dec. 3, 1940 2,286,237 ShawJune 16, 1942 2,287,082 Bauer June 23, 1942 2,359,401 Wulff oct. 3, 19442,389,734 Mehl Nov. 27, 1945 2,464,163 Balke Mar. 8, 1949 2,464,889 Pikeet al Mar. 22, 1949 FOREIGN PATENTS Number Country Date 549,954 GreatBritain Dec. 15, 1942 316,748 Germany Dec. 5, 1919 OTHER REFERENCESChemical and Metallurgical Engineering, vol. '26, (1922), pp. 639, 640,641.

1. THE PROCESS FOR MAKING BRITTLE, POROUS IRON PLATE BYELECTRODEPOSITION WHICH COMPRISES, MAINTAINING IN A CELL A SOLUTIONCOMPOSED SUBSTANTIALLY ENTIRELY OF FERROUS CHLORIDE AND WATER AT ACONCENTRATION WITHIN THE RANGE 20 TO 125 GRAMS OF IRON PER LITER AND ATA PH FROM 3 TO 5.5, SUBJECTING SAID SOLUTION TO ELECTROLYSIS BETWEEN ASOLUBLE IRON ANODE AND A CATHODE AT A CURRENT DENSITY BETWEEN 5 AND 40AMPERES PER SQUARE FOOT AND AT A TEMPERATURE WITHIN THE RANGE 15* TO 40*C., THE VALUES WITHIN SAID RANGES BEING SELECTED TO PRODUCE A BRITTLE,DULL GRAY DEPOSITE HAVING SPECIFIC GRAVITY FROM 6.3 TO 7.25.